You can but I don't think you should. For one, one manufacturer's S flex is maybe another's R. Also, some JDM shafts (not many) are stiffer than "international models".
One good example to illustrate these 2 points is the Crazy 9 line which plays stiffer to flex compared to many other brands and popular US brands as well.
You may want to try to use CPM as a better guide and even then , how one brand measures CPM may be different to how another brand measures CPM. Like flex, there is unfortunately no standard which all follow.
Only 1 sure way to find out...